Generally, when steel sheets are formed, liquid wax, lubricating oils, organic high polymers etc. are coated on the steel before forming in order to prevent rupturing and scuffing during the forming. In the above-mentioned methods, complicated processes such as a cleaning and drying are required after forming in order to remove lubricants and to make matters worse, the corrosion resistance of the part which is subjected to forming decreases considerably; however, no better method is presently known.
On the other hand, plating of steel sheets is sometimes used to prevent the decrease of corrosion resistance after forming. For instance, galvanized steel sheet and electrogalvanized sheet are widely used because the zinc plating on sheets gives a remarkable corrosion resistance to the base steel, however, zinc-plated steel sheets are not satisfactory in formability, for some cracks occur in drawing ratio 2 without coating of lubricating oil. Also in a lower drawing ratio than the 2.0, the plated layer in the part to be formed is destroyed to a great extent and consequently corrosion resistance deteriorates considerably to almost the same level of the base steel sheets without the plated layer. However, the occurence of cracks during drawing can be somewhat reduced by coating the electrogalvanized sheets with machine oil or press oil during drawing. In this case, an oil coating process and a cleaning process are required and these processes contribute to low productivity. Moreover, the corrosion resistance at the drawn part deteriorates noticeably.
These methods are referred in published Japanese patent application Nos. Sho 45-5130 and 49-24789, which applications are the starting point of the present invention.
Published Japanese patent application No. Sho 45-5130 discloses a method of coating steel sheets with solution which mainly consists of lithium silicate in order to increase corrosion resistance, however, this method provides no improved effects on formability of the sheet, and in fact, the sheet by this method requires lubricating oil during deep drawing because of its poor formability.
Published Japanese patent application No. Sho 49-24789, discloses a method to improve formability of metal plates and to prevent occurrance of defects during forming, by first coating a water glass film on a steel sheet and then coating with a solution of solid wax dissolved in an organic solvent, then followed by drying.
This method of making a dual layer by two coatings is very complicated and the corrosion resistance after forming is merely temporary, i.e. it only remains until the next surface treating process, because it is designed so that the coated films can be easily removed by hot water and not be permanent.
On the contrary, the present invention avoids these defects arising from forming of steel sheets.
The application of the present invention omits the processes of oil coating and cleaning, and forms a strong coating film which contains lithium compound, so that a steel sheet with the film of the present invention can be formed without reducing the corrosion resistance after forming. Moreover, a point of advantage of the present invention is the possibility of handling the steel sheet without contaminating the surface with the dirt, i.e. fingerprints because the film formed by the method of the present invention is solid and difficult to dissolve. Further, the solution used in the present invention is water-based, and has an advantage in safety and sanitation.